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Originally Posted by /usrs/src/linux/Documentation/laptops/laptop-mode.txt

Laptop mode is controlled by the knob /proc/sys/vm/laptop_mode. This knob is present for all kernels that have the laptop mode patch, regardless of any configuration options. When the knob is set, any physical disk I/O (that might have caused the hard disk to spin up) causes Linux to flush all dirty blocks. The result of this is that after a disk has spun down, it will not be spun up anymore to write dirty blocks, because those blocks had already been written immediately after the most recent read operation. The value of the laptop_mode knob determines the time between the occurrence of disk I/O and when the flush is triggered. A sensible value for the knob is 5 seconds. Setting the knob to 0 disables laptop mode.

Why would a certain application use it on demand? Other apps would wake up the disk like they used to. I don't think that would make a lot of impact.

I doubt this is going to go mainline. On the other side, it would be really cool to work on the Linux kernel as your thesis like it's author.

We both were wrong, but now i think that this thing implements a great idea.

Cryogenic APIs works with EVERY device that does I/O operations, not just disks, ssd or block devices in general.
The idea is that a task can decide to delay an i/o action if the i/o device is sleeping to maximize his sleeping time in the long run.
If more tasks need to access a device that is in low power state (sleeping), they both can decide to enter in a queue and perform their actions when the device is woken up for whatever reason (maybe another task that urgently needed to perform an io operation).